Secondary electron emitter



Nov. 21, 1950 L. E. CHEESMAN ET AL 2,

SECONDARY ELECTRON EMITTER Filed April 2, 1949 FIG. I

awman 0x10:

saunas I49 SOURCE SECONDARY EMISSION RAT/0 1 l y l O 40 800 I200 I6002000 2400 2800 TARGET VOLTAGE L E. CHEESMAN "35f H. 5. MENDENHALLPatented Nov. 21, 1950 2,530,947 v SECONDARY ELECTRON EMITTER Leonard E.Checsman, Clifton, and Hallam E. Menclenhall, Summit, N. 3., assignorsto Bell Telephone Laboratories, Incorporated, New York, N. Y., acorporation of New York Application April 2, 1949, Serial No. 85,162 7Claims. (CL 250-174) This invention relates to secondary electronemitters and to methods of making such emitters.

One object of the invention is to obtain enhanced secondary emissionfrom targets in electron discharge devices. More particularly, oneobject of this invention is to obtain a stable, readily fabricatedtarget having a large secondary emission ratio at relatively low primaryelectron or target voltages.

The invention involves the discovery that cadmium oxide, which normallyhas an emission ratio of the order of unity even at primary electronvoltages of several thousand volts,can be activated to have a largeratio at low voltages,

In one illustrative embodiment of this invention, a target electrodeforelectron discharge devices comprises a film of cadmium oxide, of theorder of 5,000 angstroms thick, upon a base of commercially pureplatinum. The oxide is activated by heating the film-base unit for aprolonged time, whereby the emission ratio of the oxide is greatlyenhanced. For example, in one exemplary construction, after heating ofthe unit at about 800 C. for about 22 hours, emission ratios of theorder of 4.8 at a primary voltage of 500 volts are attainable for theoxide.

The invention and the features thereof will be understood more clearlyand fully from the following detailed description with reference to theaccompanying drawing, in which:

Fig. 1 is a diagram of an electron discharge device illustrative ofthose in which secondary electron emitters constructed in accordancewith this invention may be utilized; and

Fig. 2 is a graph showing the relation between the secondary electronemission ratio and primary electron voltage, for a target electrodeillustrative of one embodiment of this invention.

Referring now to the drawing, the electron discharge device illustratedin Fig. 1 comprises a highly evacuated enclosing vessel I having thereina target electrode II and an electron gun for projecting an electronstream to and against the target electrode. For simplicity ofillustration, the electron gun has been shown as comprising anindirectly heated cathode l2 and accelerating electrode l3, it beingunderstood that any one of a number of conventional gun constructionsmay be employed such as guns including tungsten or thoriated tungstenfilamentary type cathodes. Opposite the target electrode I I is acollector electrode I l, for example cylindrical is shown, for receivingsecondary electrons emanating from the target electrode II.

The target electrode is maintained at a positive potential relative tothe gun by a direct-current source l5, the potential being variable, ifdesired, to control the energy of the primary electrons impinging uponthe target electrode. The latter is biased negative relative to thecollector electrode M by a direct-current source l6. An output or loadelement I? is connected between the target and collector electrodes asshown. The electrode I3 is biased positive relative to the cathode by asource IS.

The target electrode I l comprises a commercially pure platinum basehaving on the face thereof toward the electron gun a layer or film ofcadmium oxide. Such a layer or film may be produced in one way bycathodic sputtering of cadmium upon the base, operated as the anode, inan oxidizing atmosphere. In an illustrative case, a film. of the orderof 5,000-angstrom units thick maybe'deposited in about four hours bycathodic sputtering from a flat spiral cadmium rod spaced about twoinches from the platinum base, and in a partial vacuum such that with adirect-current voltage of 400 volts applied between the cathode andanode through a 5,000-ohm ballast resistance the cathode dark space ofthe discharge extends to within about one-half inch from the base to becoated. After the base has been coated, the composite unit may be heatedin a hydrogen oven at 200 C. for about two hours whereby an increase inthe conductivity of the coating is realized.

The secondary emission characteristic of cadmium oxide treated inaccordance with this invention Will be appreciated from Fig. 2. In. thisfigure, curve A depicts the secondary emissionprimary electron voltagerelation at room temperature for a target electrode constructed asdescribed above, the cadmium oxide coating being of the order of 5,000angstroms thick and the base being of commercially pure platinum. As isevident, the secondary emission ratio is quite low, having a maximumvalue of about 1.4 at a bombarding voltage of about 600 volts. It hasbeen determined that with the target maintained at temperatures up to700 0., by passing heating current ther-ethrough from a source H! asillustrated in Fig. 1, the secondary emission ratio is uniformly low,the maximum obtained being somewhat above 2. 1

However, when the target electrode is heated to about 800 C. thesecondary emission ratio increases suddenly. Curve B in Fig. 2 shows thesecondary emission characteristics of the target, at room temperature,after the target had been heated at 800 C. for 21 hours and then at 900C.

for one hour. The maximum secondary emission ratio, it will be noted, isabout 4.8 at a primary electron voltage of about 500 volts.

Although the invention is not to be limited thereby, the followingexplanation is thought to account for the enhanced secondary emission ofthe cadmium oxide film obtained :by heating the target. The temperature,about 800 0., of treatment at which a marked secondary emissionenhancement obtains is about that at which, in vacuum, cadmium oxidebegins to .break .down to release cadmium. The cadmium atoms thusreleased activate the oxide film. Furthermore, commercially pureplatinum contains minute quantities of impurities of the "order ofmagnitude indicated below.

Per cent Tin and palladium 0.01 to 03 Copper less than 0.03

out departing irom the scope and spirit of this invention.

What is claimed is:

-1.. The method of "making a secondary electron emitter whichcomprisesapply ing a layer of sadmium oxide to a base, and heating thecomposite body thus formed at a temperature of about 800 C.

2. The method of making a secondary electron emitter which comprisesapplying a film of cadmium oxide to a base of platinum containingimpurities, and heating the composite body thus formed at a temperatureof the order of 800 C.

3. The method of making a secondary electron [emitter which comprisesapplying to a base of commercially .pure platinum a layer of cadmiumoxide, of the order of 5,000 angstroms thick, and heating the compositebody thus formed at *attemperature .of about 800 C. for about 21 hours.

'4. The method defined in claim 3 comprising -further heating said bodyat a temperature of about 900 C. for approximately one hour.

5. A secondary electron emissive target for electron discharge devicescomprising a base having thereon a film of cadmium oxide.

6. .Asecondaryelectron emissive target forelectron'discharge devicescomprisinga base of commercially pure platinum having thereon a :film ofcadmium oxide, the base-oxide body having been lheated at a temperatureof the order of 800 7.. .Asecond-ary electron emissive target forelectron discharge devices comprising a base of :commercially pureplatinum having thereon ;a 'film of cadmium oxide of the :order of 5,000angstroms thick, the base-oxide body having been heated :at atemperature of about 800 C. for approximately 21 hours and at atemperature of about 900C. for approximately one hour.

LEONARD :CHEESMAN. .I-IALLAM *E.

No references cited.

5. A SECONDARY ELECTRON EMISSIVE TARGET FOR ELECTRON DISCHARGE DEVICESCOMPRISING A BASE HAVING THEREON A FILM OF CADMIUM OXIDE.